The variety of distinct publishing systems in computing devices that generate graphics will invariably specify color space data that is not always optimal for a particular printer or display device. The colors of graphical objects produced by such desktop publishing software will vary between different types of display devices. The rendering of job specified color space data into display device color space data will often necessitate assumptions by the display device that can produce unsatisfactory results for the system operator. The subject invention will minimize the problems engendered by such assumptions.
Graphics colors can be described in any variety of color systems or color spaces. Note PostScript Language Reference Manual, 2nd edition, Section 4.8, "Color Spaces", pages 176-200, 1990, Addison-Wesley Publishing Co. Most desktop publishing systems will endeavor to specify abstract colors in a device independent way. But the color values that originate from an application generating a source page description language file can be defined in a number of different color spaces. Some color spaces are related to device color representation (gray scale, RGB and CMYK), or can be related to device independent representations, e.g., XYZ, CIELAB. However, some independent color spaces still have a device-bias that will cause generation of unsatisfactory results if merely passed through for display. It is the problem of whether and how to overload such independent but device-biased color spaces that this subject invention addresses.
The conventional rendering facilities of a printing or display system will utilize a set of transformations to convert the input color spaces as specified by the job to a device color space for such systems, in an effort to be adaptable to different input color spaces and achieve consistent color output or intended rendering of the output. It is common to have various pathways through the rendering system with different capabilities, such as a device path or a calibrated path. The device path communicates input device colors and renders them as output device colors. The calibrated path is a more powerful path and generally has more sophisticated processing associated with it than the device path. The calibrated path facilitates calibrating by the system of the input color space to get a more consistent output. The Xerox Intelligent Color.TM. (pending application Ser. No. 08/664,297, filed May 19, 1996) rendering system takes advantage of the processing power of a calibrated pathway by forcing the calibrated path through a remapping of some of the originally input color spaces. Color space remapping may also take place in the calibrated path, in order to ensure that it is optimized for a particular output device.
Compatibility between the original color space and the overloading color space of the Xerox Intelligent Color.TM. rendering system must be ensured in order to avoid serious rendering defects, such as images which are rendered as black.
Accordingly, a first problem with systems that force overloading through a rendering processor exists in the limitation of the printer to require a compatibility between the input original color space and the overloading color space which, of course, limits the adaptability of the printing system.
A second problem is that even though there are some colors, like device colors, that are desirable to process through the calibrated path, there are other colors that are processed on the calibrated path that are a little more free formed and which should be passed through the rendering system without processing for overloading. In particular, in a Postscript system, CIE based ABC is an abstract color space which, depending on color space parameters, may be used to implement a number of color spaces including CIELAB and other device-biased color spaces. If the original input color space is CIELAB, it is desirable to have such a color space pass through the rendering system. On the other hand, if the input color space is a calibrated RGB space that is set for a particular monitor, that color space is usually desirable for overloading in the rendering system of a printer.
Another problem occurs where a calibrated RGB color space has an incompatible gamma for a selected printer. Unsatisfactory outputs, such as dark prints, might result if calibrated RGB color space with gammas that are incompatible with the rendering system are processed in the usual fashion.
The present invention contemplates a new and improved system and method for rendering a job specified input color space which overcomes the above problems and provides a selective color space overloading system which is readily adaptable to a plurality of original input color spaces and which can provide improved consistency and satisfactory results in displayed or printed graphics outputs.